1. Field of the Invention
The present invention relates to a method for manufacturing a thin film of metal-oxide dielectric used in a memory device, a pyroelectric device, a piezoelectric device, an electrooptic device, etc.
2. Description of the Related Art
Various kinds of applications of a composite metallic oxide dielectric are generally considered by using its electric characteristics in a field of electronics. For example, the composite metal-oxide dielectric is used in various kinds of devices such as an infrared linear array sensor using a pyroelectric property, a supersonic sensor using a piezoelectric property, a waveguide type optical modulator using electrooptic effects, a capacitor for a DRAM and a MMIC (Microwave Monolithic IC) using a high dielectric property, etc.
In particular, the area of a capacitor having a large capacity is reduced by using a material having a high dielectric constant in an integrated circuit (IC), such as GaAsMMIC, in which this capacitor is built.
For thin film material having such a high dielectric constant, a composite metal-oxide dielectric such as SrTiO.sub.3 (strontium titanate), BST (barium strontium titanate), PLZT (lead lanthanum zirconate titanate), etc is used. A BST material is vigorously researched as a most promising material at the present time. When this thin film is practically used, the thin film of metal-oxide dielectric is desirably manufactured at a low temperature in a manufacturing process to improve temperature characteristics and adapt this manufacturing process to processes for manufacturing various kinds of semiconductor elements.
A manufacturing method of the thin film of metal-oxide dielectric uses a physical method such as a vacuum evaporation method, a sputtering method, a laser abrasion method, a chemical method such as a sol-gel method, a CVD (Chemical Vapor Deposition) method, etc. In the sol-gel method, organometal compounds are set to raw materials for starting and the thin film of metal-oxide dielectric is obtained by thermally decomposing these raw materials.
The sol-gel method is widely used in the manufacturing method of the thin film of metal-oxide dielectric since the raw materials can be uniformly mixed with each other at an atomic level and the dielectric thin film can be industrially manufactured at low cost.
A general method for manufacturing a BST thin film with respect to the sol-gel method will next be explained.
First, organometal compounds, such as alkoxides of barium, strontium and titanium, etc. are mixed with each other and are adjusted in a suitable solvent such as ethanol, propanol, butanol, methoxyethanol, acetic acid, etc. Thereafter, a thin film of a BST composite metal-oxide dielectric is manufactured in the following manufacturing processes.
(1) The organometal compounds are heated and stirred in an inert gas atmosphere of nitrogen, etc. to prepare a precursor solution of sol of composite alkoxide having a two-dimensional chain structure as shown in the following chemical formula 1. ##STR1##
(2) This sol (the precursor solution) is applied on a substrate so as to form a thin film by a spin coating method, a dip coating method, etc. In this process, the sol of the two-dimensional chain structure is hydrolyzed by moisture in the air and becomes gel of a three-dimensional network structure as shown by the following chemical formula 2 by a condensation polymerization reaction subsequently caused. ##STR2##
(3) The obtained gel formed in a film shape is heated and dried to separate alcohol reactively produced in the process (2) and the residual moisture not absorbed into the network structure from the film.
(4) The film is then subjected to a heat treatment at a temperature in a range from 350.degree. C. to 450.degree. C. to convert this gel state film into amorphous by thermally decomposing and removing organic components in the film.
(5) The film in the amorphous state is subjected to another heat treatment at a temperature in a range from 650.degree. C. to 750.degree. C. to crystallize this film.
The processes (2) to (4) are repeated plural times before carrying out the process (5) to obtain the thin film of a BST composite metal-oxide dielectric having a desirable thickness.
Thus, the thin film of the BST composite metal-oxide dielectric can be manufactured.
However, for example, it is necessary to set a film forming temperature to be equal to or lower than 600.degree. C. for maintaining transistor characteristics in a Hetero-junction Bipolar Transistor (HBT) structure used in GaAsMMIC.
In the above general manufacturing method of the thin film of metal-oxide dielectric, it is difficult to provide a single orientation for the thin film of metal-oxide dielectric manufactured through the processes (4) and (5), i.e. , manufactured by carrying out the heat treatment for crystallization after the process for converting the thin film into amorphous. Accordingly, the thin film is orientated at random. Further, for obtaining the thin film of metal-oxide dielectric having a high dielectric constant, it is necessary to heat the thin film at a high temperature in a range from 650.degree. C. to 750.degree. C. in crystallizing process such as the process (5) in the general manufacturing method of the thin film of metal-oxide dielectric.